Composition for treating water with resin bound ionic silver

ABSTRACT

A composition is provided for treating drinking water for disinfecting and/or removing iodide. The composition utilizes resin bound silver ions. For performing the disinfection or iodide removal with minimal release of silver ions into the water being treated, a chelating resin having iminodiacetate chelating groups is employed, and the resin is loaded with not over 0.5 mol of silver ions per mol of iminodiacetate.

RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.08/210,876, filed Mar. 18, 1994, now U.S. Pat. No. 5,366,636.

FIELD OF INVENTION

The field of this invention is the treatment of water with silver todisinfect the water and/or remove halide therefrom. The invention isparticularly concerned with the treatment of drinking water to removeiodide.

BACKGROUND OF INVENTION

It is known that metallic silver can be an effective bactericide fortreating water. For this purpose U.S. Pat. No. 2,434,190 describes thepreparation of a "silvered" anion exchange resin. As described in thispatent, the anion exchange resin in the sodium form may be treated witha solution of silver nitrate to load the resin with silver ions. Theresin is next treated with a reducing agent, such as potassiummetabisulfite, to form metallic silver. It is stated that the silveredresin can be used for disinfecting water.

U.S. Pat. No. 2,692,855 discloses using a cation exchange resin in asilver ion form for disinfection of water. As described in the patent,for example, a cation exchange resin In the hydrogen form may be treatedwith a solution of a silver salt to exchange silver ions for hydrogenions, thereby producing a resin for disinfecting water. However, thepresence of silver ions in drinking water can be a health hazard, anddrinking water contains metal ions that will exchange with the silverions on the resin. To minimize release of silver ions into the solutionbeing treated, the water can first be demineralized by passing itthrough an cation exchange resin in the hydrogen form to remove metalcations.

Cation exchangers, such as zeolite and synthetic cation exchange resins,have been reacted with solutions of silver salts to exchange the silverion for the hydrogen or alkali metal cations of the exchangers. Suchsilver ion-containing resins have been used to remove halides fromwater, such as the removal of chloride ions from sea water. See U.S.Pat. No. 3,32,039, United Kingdom Patent 576,969 and Australian Patent122,647. Silver ion-containing cation exchange resins have also beenproposed for use in removing iodine and methyl iodide from wastestreams, and removing halides from liquid carboxylic acid (U.S. Pat. No.5,139,981). In such applications, the release of silver ions into thesolution being processed may not be as objectionable as with potablewater.

SUMMARY OF INVENTION

Prior art methods of using silver ion-containing zeolites and syntheticcation exchange resins to treat drinking water have the disadvantage ofreleasing silver ions into the treated water. Drinking water containscations such as calcium, magnesium, and sodium which tend to exchangewith the silver ions in the zeolite or resin. For health reasons, it isdesired to avoid excessive amounts of silver ions in drinking water. Forexample, United States Environmental Protection Agency established as asafety standard that drinking water should contain less than 50 μg(micrograms) of silver per liter of water. In treating drinking water todisinfect the water and/or to remove iodide therefrom it is thereforeimportant to carry out the treatment with minimized release of silverions.

The present invention utilizes a novel resin composition which comprisesa chelating resin containing iminodiacetate acid groups at the metalchelating sites, which provide paired acetate groups and a tertiaryamine for chelating action. The resin composition is furthercharacterized by containing diacetate-chelated silver ions in a ratio ofnot over 1 silver ion per two iminodiacetate groups. The resin isthereby loaded with silver ions to not over 50% of resin capacity. Thesilver ions are thereby retained within the resin beads whileeffectively killing microorganisms, and/or removing iodide from thewater. The resulting resin beads tenaciously retain the silver ions andresists their elution by the cations normally found in water, such ashydrogen, sodium, calcium, and magnesium ions.

The method of this invention also utilizes the retaining capacity of thesilver chelating resin composition to minimize the release of silveriodide, silver chloride, or other silver halide into the drinking water.Although the exact mechanism of retention is not fully understood, ithas been found that the soluble silver iodide or other silver halideformed within the resin granules or beads are retained therein,resulting in the production of drinking water with minimal content ofiodide, silver ion, or silver iodide.

The method of this invention can be used as a desirable secondarytreatment for drinking water which has been contacted with porousgranules of a polyiodide anion exchange resin to disinfect the water.Such treatment may release iodide ions into the drinking water. In onepreferred embodiment, the drinking water has initially been disinfectedby being contacted with an anion exchange resin containing pentaiodide(I₅ ⁻) ions.

DETAILED DESCRIPTION

For preparing the silver ion containing chelating resin composition, aresin containing iminodiacetate groups is used. Bio-Rad Laboratories,Richmond, Calif., sells such chelating resins which are polystyrenedivinyl benzene copolymers containing iminodiacetate functional groups.These resins are identified as "Chelex 20" (macroporous form) and"Chelex 100" (gel form). Either the gel or macroporous form can be used,but the gel form Chelex resin is preferred. Chelex 100 is available inanalytical and biotechnology grades both of which are suitable for usein this invention. Chelex 20 is a technical grade resin which can alsobe used.

The described chelating resin is contacted with an aqueous solution of asilver salt, such as the nitrate, perchlorate, or acetate salts. Thesilver ions are removed from the solution and immobilized by the pairedchelating groups. This conversion to a silver form should be carried outin relation to the stated cation capacity of the resin, viz. inmillieqivalents per milliliter. The resin composition is prepared sothat it does not contain silver ion which easily exchange and/or elute.The quantity of silver ions applied to the chelating resin should notexceed one silver ion per two iminodiacetate groups, which correspondsto a 50% or less capacity loading. Stated otherwise, the prepared resincomposition should contain not over 0.5 mol of silver per mol ofiminodiacetate. Resin compositions can also be used which contain lessthe stated maximums of silver, such as 0.3 to 0.5 mol of silver per molof iminodiacetate.

Preparation of Silver-Chelex 100 Resin

Approximately 300 milliequivalents of Chelex 100, a gel-type chelatingresin (50-100 mesh or 100-200 mesh, sodium form; Bio-Rad Laboratories,Richmond, Calif.) is suspended in an excess of distilled water. Thesettled wet capacity of this resin is 0.40 meq/ml. This resin containsiminodiacetate chelating groups. Total volume is approximately 750 ml ofsettled bed volume. The pH of the aqueous suspension is measured with astandard glass electrode and adjusted with 1.0 N NaOH to at least pH 8.0if required. Most commercial lots of the resin will generate asuspension with pH >8.0 but an occasional lot may requirestandardization. It is believed important to open up both of the acetategroups on the iminodiacetate to accept the silver cation, Ag⁺.

A solution of silver nitrate (AgNO₃) 150 milliequivalents (25.48 grams)in 200 ml distilled water is added to the suspended Chelex 100. Themixture is stirred with an overhead glass stirring rod to prevent beadfracture. After one hour the stirrer is turned off and the gel allowedto settle. The bed volume typically shrinks to 500 ml due toneutralization of the electrorepulsive effects of the adjacent diacetategroups.

The supernatant solution is tested with 0.10M KI solution and producesno silver iodide precipitate. The free silver ion concentration of thesupernatant solution is undetectable with a silver select ion electrode(Ag⁺ <1×10⁻⁶ M). If more than 0.5 equivalent Ag⁺ :equivalent Chelexresin is used in preparation an extensive washing procedure is requiredto eliminate the free silver ion in aqueous washes.

Preparation of Silver-Chelex 20 Resin

Approximately 300 milliequivalents of Chelex 20, a macroporous chelatingresin (20-50 mesh, sodium form; Bio-Rad Laboratories, Richmond, Calif.)is suspended in an excess of distilled water. This resin containsiminodiacetic acid-type chelating groups. Total volume is approximately500 ml of settled bed volume. (This resin requires only 500 ml toprovide 300 meq of binding capacity.) The pH of the aqueous suspensionis measured with a standard glass electrode and adjusted with 1.0 N NaOHto at least pH 8.0 if required. Most commercially available lots of theresin will generate a suspension with pH >8.0 but the occasional lotrequires standardization. This is important to open up both of theacetate groups on the iminodiacetate to accept the silver cation, Ag⁺.

A solution of silver nitrate (AgNO₃) 150 milliequivalents (25.48 grams)in 200 ml distilled water is added to the suspended Chelex 20. Themixture is stirred with an overhead glass stirring rod to prevent beadfracture. After one hour the stirrer is turned off and the gel allowedto settle. The bed volume typically shrinks to 450 ml which is probablydue to neutralization of the electrorepulsive effects of the adjacentdiacetate groups.

The supernatant solution is tested with 0.10M KI solution and producesno silver iodide precipitate. The free silver ion concentration of thesupernatant solution is undetectable with a silver-selective ionelectrode (Ag⁺ <1×10⁻⁶ M). If more than 0.5 equivalent Ag⁺ :equivalentChelex resin is used in preparation an extensive washing procedure isrequired to eliminate the free silver ion in aqueous washes.

An experimental investigation was conducted with the resins prepared asdescribed above.

First Experiment

Escherichia coli B, strain NP 4, was grown overnight in Benzer Broth(9.0 gm Bacto Tryptone [Difco] and 5.0 gm NaCl per liter) in a shakingwater bath (New Brunswick) at 37° C. Organisms were centrifuged at10,000×g for 5 minutes in a Beckman J-21 centrifuge at 4° C.,resuspended in deionized water, washed, centrifuged and diluted 1:100 indeionized water to give approximately 1×10⁷ colony forming units per ml(cfu/ml). These organisms were allowed to flow through beds of thesilver resins.

Samples of the water were plated on nutrient agar plates (Benzer Brothsolidified with 15 gm Bacto Agar [Difco] per liter); 0.10 ml sampleswere spread with a sterile glass rod. Platings were done in triplicate.As a control the input culture was appropriately diluted (10⁻⁵) and alsoplated in triplicate as described above. Plates were incubated for 16hrs. at 37° and colony forming units determined. Data are expressed inthe following table.

    ______________________________________                                        Bacterial Concentration E. Coli (CFU/ML)                                      Trial              Input     Output                                           ______________________________________                                        Silver-CHELEX 20 Resin                                                                           2.79 × 10.sup.7                                                                   0                                                Silver-CHELEX 100 Resin                                                                          2.79 × 10.sup.7                                                                   0                                                ______________________________________                                    

This data illustrates the bactericidal activity of the silver resincompounds. The data is surprising in light of the tenacity with whichthe silver ion are held by the iminodiacetate resin. Further, tests ofthe resin eluates with a solution of 1×10⁻¹ M KI formed no visibleprecipitate, indicated no significant elution of the silver ion, Ag⁺.

Second Experiment

An aqueous solution of potassium iodide (KI) 1×10⁻⁵ M which contains1.27 mg per liter of iodide anion (I⁻) was passed under gravity flowthrough a small 5 ml column of the three silver ion-containing resinmaterials prepared as described above. The iodide anion concentrationwas determined using an iodide-selective electrode to record theelectrode potential. The electrode potential was converted to iodideconcentration by reference to a standard curve. The results of fourtrials are reported in the following table.

    ______________________________________                                        Iodide (I.sup.-) Removal by Silver Resins                                              Input    Solution ppm                                                                             Output Solution ppm                              Trial    millivolts                                                                             (mg/l) I.sup.-                                                                           millivolts                                                                           (mg/l) I.sup.-                            ______________________________________                                        Ag-Chelex 20                                                                           +120     1.27       +490   <.00127                                   Ag-Chelex                                                                              +120     1.27       +545   <.00127                                   100                                                                           ______________________________________                                    

In both trials the iodide anion concentration in the eluates was lessthan the lowest concentration that could be detected with theiodide-selective electrode (220 millivolts: 0.00127 ppm).

Preferred Combination Treatment

In a preferred application of the method of this invention, the water tobe treated is first passed through a quaternary ammonium exchange resin,which as first used has more than sixty-five percent of the ion exchangesites therein associated with pentaiodide ion (I₅ ⁻). For example, aresin of this kind can be prepared as described in the example of U.S.Pat. No. 4,999,190. The resulting resin will have about ninety-sevenpercent of its total sites iodinated and about seventy percent of thesites will be I₅ ⁻ sites. The water to be disinfected is first passedthrough a bed containing granules of this resin. Bacteria and othermicroorganisms will be killed and the treated water will contain iodideions (I⁻). To assure complete disinfection and to remove the iodideions, the initially treated water is passed through one of the silverion-containing resins prepared as described above. The combinedtreatment will produce bacterially sterile water substantially free ofiodide and silver ions. To assure that the treated water complies withthe EPA standard of 50μ230 g silver/l, activated charcoal may be mixedwith the silver chelating resin or used as a tertiary treatment.

Comparative Example Silver Chelex 20

Silver Chelex 20 and 100 resins were prepared as described abovecontaining 0.5 mol of silver ion per mol of iminodiacetate. A sulfonicacid resin was loaded with silver ions to 50% capacity and three zeoliteexchangers were loaded at less than their maximum capacities. Thepreparation procedure is described below.

Preparation of Silver Sulfonic Acid Resin

Approximately 300 milliequivalents of AG 50W-X8, a strong cationexchange resin (20-100 mesh, hydrogen form; Bio-Rad Laboratories,Richmond, Calif.) is suspended in an excess of distilled water. Totalvolume is approximately 500 ml of settled bed volume.

A solution of silver nitrate (AgNO₃) 150 milliequivalents (25.48 grams)in 200 ml distilled water is added to the suspended AG 50W-X8 resin.(This corresponded to 0.5 mol silver per mol sulfonate.) The mixture isstirred with an overhead glass stirring rod to prevent bead fracture.After one hour the stirrer is turned off and the gel allowed to settle.

Preparation of Silver Zeolites

Zeolite cation exchange material (Fisher Chemical Company) was utilizedin three commercially available forms which are marketed as molecularsieves; Type 5A, in 1/16 inch pellets; Grade 512, in 4-8 mesh beads; andGrade 513, in 4-8 mesh beads. These zeolites were composed of aluminasilicate with either sodium or calcium cations.

The three zeolites were "converted" to the silver cation form bysuspending 100 cm³ of each material in 100 ml of 0.10M silver nitrate(AgNO₃) solution for 24 hr. at room temperature. The total volume ofeach of the silver nitrate-zeolite preparations was in excess of 150 ml.After overnight reaction some darkening of the solution occurred. After24 hr. the excess AgNO₃ was decanted and the zeolite washed three timeswith borosilicate-glass distilled water.

Column Experiments

All six materials (two Chelexes, the silver sulfonic acid resin andthree zeolites) were individually paced into 20 ml syringes with fiberglass plugs. Distilled water, Manhattan, Kans., city tap water(approximately 300 ppm total dissolved solids), or tapwater spiked with1×10⁻⁵ M KI was allowed to flow through the resin beds. Thisconcentration of KI was chosen because it is a typical concentration ofiodide anion that is encountered when polyiodide anion exchange resins(triiodide or pentaiodide) are used in the chemical disinfection ofwater. Column eluates were then tested for free silver or iodide ions(Ag⁺ or I⁻) using appropriate ion selective electrodes and reference toa standard Nernst equation relating a millivolt reading to therespective ion concentration.

The results of these tests are summarized below in Tables 1 and 2.

                  TABLE 1                                                         ______________________________________                                        Free silver ion concentrations of eluates from                                small columns of silver resins using distilled water.                         RESIN MATERIALS   [Ag.sup.+ ] M                                               ______________________________________                                        Ag-Chelex 20, 100 0.9 × 10.sup.-5                                       Ag-Sulfonic Acid  1.3 × 10.sup.-3                                       Ag-Zeolite 5A     1.1 × 10.sup.-2                                       Ag-Zeolite 512    1.7 × 10.sup.-2                                       Ag-Zeolite 513    1.6 × 10.sup.-2                                       ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Free silver ion concentrations of eulates from                                small columns of silver resins using tap water containing                     1 × 10.sup.-5 M KI ( 120 mVolts).                                       RESIN MATERIALS   [Ag.sup.+ ] M                                               ______________________________________                                        Ag-Chelex 20, 100 <1.0 × 10.sup.-6                                      Ag-Sulfonic Acid  1.2 × 10.sup.-3                                       Ag-Zeolite 5A     1.1 × 10.sup.-2                                       Ag-Zeolite 512    1.7 × 10.sup.-2                                       Ag-Zeolite 513    1.6 × 10.sup.-2                                       ______________________________________                                    

All resin materials removed the iodide, reducing I⁻ mol concentration tobelow 1.0×10⁻⁷. The important difference was that the sulfonate andzeolites released much more silver ion into the water than did theChelex 20 and/or resins. The difference was several orders of magnitude.

We claim:
 1. A composition for treating water, comprising porousgranules of a chelating resin comprising a polystyrene divinyl benzenecopolymer having iminodiacetate chelating groups with silver ions boundthereto, said bound silver ions being present in an effective amount fordisinfecting water but not over 0.5 mol of silver ions per mol ofiminodiacetate, said effective amount selected such that said silverions are retained by said chelating groups to ensure that water treatedby said chelating resin is substantially free of silver ions.
 2. Thecomposition of claim 1 in which said silver ions are present in saidgranules in an amount within the range from 0.3 to 0.5 moles of silverper mol of iminodiacetate.
 3. The compositions of claims 1 or 2 in whichsaid chelating resin is a gel-type resin.
 4. A composition for treatingdrinking water to disinfect and/or remove iodide therefrom, comprisingporous granules of chelating resin comprising a polystyrene divinylbenzene copolymer having iminodiacetate chelating groups with silverions bound thereto, said resin containing an amount of from 0.3 to 0.5mol of bound silver ions per mol of iminodiacetate, said amount selectedsuch that said silver ions are retained by said chelating groups toensure that water treated by said chelating resin is substantially freeof silver ions.
 5. The composition of claim 4 in which said chelatingresin is a gel-type resin.